The cellular industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. The number of cellular users in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of the rapid growth will soon be achieved even in the smallest markets. Innovative solutions are thus required to meet these increasing capacity needs as well as to maintain high-quality service and avoid raising prices. Furthermore, as the number of cellular users increases, the problems associated with co-channel interference become of increased importance.
Current digital cellular systems employ base stations which separate signals from mobile stations using time and frequency orthogonality. Signals from a mobile propagate to a base station and the signals are received in a single or sometimes double antenna. The receiver processes the signal using time and frequency orthogonality to separate signals from different users. It is then possible to equalize and detect the signals. While techniques such as frequency hopping and advance coding techniques provide ways to reduce the effects of co-channel interference, they are inherently limited by the available frequency spectrum. However, the use of the directional sensitivity of adaptive antenna arrays offers a new way of reducing co-channel interference. An adaptive antenna array consists of an array of spatially distributed antennas. Signals from several transmitters converge on the array from various directions. By properly combining the antenna outputs, it is possible to extract individual signals from the received superposition, even if they occupy the same frequency band. It is then possible to distinguish between spatially separated users by using narrow adaptive antenna lobes. This can be viewed as a way to utilize orthogonality in the spatial dimension.
The use of antenna arrays implies that the detector structure in a receiver must be modified. Temporal and spatial symbol interference suggests the possibility of using power from various directions in a constructive way. Spatially separated signals can not simply be added due to the temporal symbol interference. There is thus a need for a joint detection of various propagation paths. A "joint maximum likelihood sequence estimator" (MLSE) solution has been proposed for the joint detection of all mobiles using a channel. The joint MLSE solution can be implemented as a channel identification followed by a multi-input Viterbi detector, where the output of the detector is the detected data for a mobile station. However, the joint MLSE solution is prohibitively complex. The complexity grows very fast with the number of elements in the array and with the number of mobiles to be jointly detected.
Another receiver structure is disclosed in "An Adaptive Array for Mobile Communication Systems," S. Andersson et at., IEEE Trans. on Veh. Tech., vol. 40, no. 1, pp. 230-236 (Feb. 1991), in which a spatial filter is optimized by various methods and the performance is evaluated by the signal-to-noise ratio. However, this article does not discuss transmission nor the problems associated with intersymbol interference.
Yet another receiver structure is disclosed in "Optimum Combining in Digital Mobile Radio with Cochannel Interference," J. H. Winters, IEEE Trans. on Veh. Tech., pp. 144-155 (Aug. 1984), wherein the spatial filter is trained using detected data and a Least-Mean-Squares algorithm. Winters proposes that the weights from the receiver filter are also used for transmission. However, this article assumes flat fading, i.e., no intersymbol interference so that the data can be detected using one spatial filter followed by a quantizer.
Another proposed receiver solution suggests the use of a "spatial demultiplexor" using a spatial filter, computation of relative delays and coherent combination. However, this receiver solution can not handle temporal multipaths from one direction. As a result, this solution would not be feasible in systems, such as GSM, where each spatial radio channel typically creates intersymbol interference over 2 or 3 symbols.